fix calReleaseNumCommitsBehind (#24148) (#24197)

Backport #24148

`repoCtx.CommitsCount` is not reliably the commit count of the default
branch (Repository.GetCommitsCount depends on what is currently
displayed).

_contributed in the context of @forgejo_

---------

Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

models/fixtures/repository.yml

@@ -1,3 +1,4 @@
+# don't forget to add fixtures in repo_unit.yml
 -
   id: 1
   owner_id: 2
@@ -1634,3 +1635,16 @@
   is_private: true
   num_issues: 1
   status: 0
+
+-
+  id: 57
+  owner_id: 2
+  owner_name: user2
+  lower_name: repo-release
+  name: repo-release
+  default_branch: main
+  is_empty: false
+  is_archived: false
+  is_private: false
+  status: 0
+  num_issues: 0